An application which exhibits an immediate need for the improved maneuverability provided by this invention is an interceptor missile. Enemy offensive missiles pose an escalated challenge for interceptor missiles. Modem threat configurations are designed to realize reduced radar signatures, make use of expanded countermeasures, travel at extremely high velocities over unpredictable or difficult to predict trajectories, and employ large magnitude lateral evasive maneuvers. In order to accomplish body-to-body impact, the interceptor missile must achieve large transverse acceleration levels in a very short period of time to move the vehicle perpendicular to its flight path to ensure collision.
As shown in FIG. 1, a missile system can be described as an elongated body 100 that travels through a fluid medium. The missile 100 has a forward section and an aft section divided by a point of center of gravity 105. Forward of the center of gravity 105 is a forward control device such as thrusters 110. The aft section has an aft control device such as fins 115. It will be apparent to one of ordinary skill in the field that other alternative control devices are possible. For instance, the forward control device could be implemented as canards rather than as thrusters. Similarly, the aft control device could be implemented via thrust-vector control techniques.
FIG. 1 shows the vehicle configuration, sign convention, and notation used in this discussion for a body fixed coordinate system allowing motion in the x-z plane. Table 1 describes the notation introduced in FIG. 1.
TABLE 1 ______________________________________ Notation Symbol Description ______________________________________ x longitudinal body fixed (right-hand) Cartesian coordinate y transverse body fixed (right-hand) Cartesian coordinate z Universe body fixed (right-hand) Cartesian coordinate N.sub.z transverse acceleration load factor along body axis z q missile pitch rate about body axis y U.sub.0 longitudinal velocity along body axis x w transverse velocity along body axis z .delta..sub.fin aft fin deflection angle .delta..sub.thr magnitude of applied thrust force ______________________________________
A missile moves in a transverse direction in response to an applied control force according to the laws of physics. Below the altitude of approximately 20 kilometers, a missile's primary source of transverse acceleration is the aerodynamic force resulting from the missile body being at an angle with its velocity vector (angle of attack). Flight control devices (e.g., forward thrusters 110 and/or aft fins 115) obtain this angle of attack by applying a moment to rotate the missile's front end in the direction of the intended maneuver.
A functional block diagram of a conventional missile control system is shown in FIG. 2. Block 200 represents the physical vehicle (i.e., the missile) and incorporates all vehicle subsystems including, for example, control actuation, propulsion and inertial measurement systems as well as aerodynamic configuration. The vehicle's measured dynamic response is shown as feedback signal 205. This signal encodes, for example, a measurement of the missile's 100 rotational and translational rates and accelerations. The missile guidance logic shown in block 210 provides a commanded dynamic response signal 215 which encodes a desired maneuver along a kinematic trajectory. The difference between the desired and measured responses produce the error signal 220 in a conventional feedback architecture. The autopilot controller 225 uses the error signal to generate a control signal 230. This control signal encodes commands to actuate the vehicle's control devices. For example, the control signal 230 could be degrees of deflection of a fin or canard, or degrees of deflection of a rocket motor nozzle, or percentage of maximum thrust of an attitude control motor, etc.
2.1 Forward Control Device Systems
One type of conventional missile control system employs a forward control device only. An example of this type of missile system is the FLAGE missile designed by LTV Aerospace Corporation (now Loral Vought Systems, the assignee of this application). The FLAGE missile employs active control of forward thrusters to achieve maneuverability. In the FLAGE missile, aft fins are fastened in a fixed canted position to provide stabilization and rolling characteristics.
A conventional control scheme employing a forward control device (e.g., thrusters) only is shown in FIG. 3. In response to a command signal 300 (corresponding to command signal 215) from the guidance system 210 for a desired step increase in lateral acceleration in the positive z-direction, the missile's autopilot controller 225 generates a time varying thruster command signal 305 (corresponding to control signal 230) to effect the maneuver. Actuation of the lateral control thrusters produce the measured acceleration response 310 in the positive z-direction normal to the vehicle's body. It is conventional to illustrate the acceleration by normalizing with the missile's weight producing a load factor N.sub.z having the units of g-force.
At time t.sub.0 guidance system output (215 and 300) commands a step increase in positive z-axis acceleration. Referring to signals 305 and 310, between times t.sub.0 and t.sub.1 the autopilot controller 225 commands the forward thrusters to deliver a force 305 in the positive z-direction to rotate the missile's nose in the positive z-direction 310 (also known as a negative pitching moment). Between times t.sub.1 and t.sub.2, the autopilot controller commands the thrusters to deliver a negative force 305 to slow the missile's downward rotation. After time t.sub.2, the autopilot controller commands a positive force 305 to hold a steady rate and acceleration in the positive z-direction 310. (Note, one skilled in the art will realize that this description also applies to accelerations in other directions.)
It is important to note that forward control mechanisms achieve missile rotation by applying a force in the direction of the maneuver, that is, ALL missile acceleration 310 is in the direction of the maneuver.
2.2 Aft Control Device Systems
Another conventional missile control technique employs an aft control device only. Examples include the Patriot missile system (Raytheon), VT-1 missile system (Loral Vought Systems) and the ATACMS missile system (Loral Vought Systems). In these systems, active control of the aft flight control surfaces (fins or thrusters) are employed to achieve maneuverability.
A conventional control scheme employing an aft control device (fins) only is shown in FIG. 4. In response to a command signal 300 (corresponding to command signal 215) from the guidance control system 210 for a step increase in acceleration in the positive z-direction, the missile's autopilot controller 225 generates a fin control signal 400 (corresponding to the control signal 230) to effect the maneuver. Signal trace 405 represents the missile's measured transverse acceleration response N.sub.z from the missile's inertial measurement system (corresponding to the feedback signal 205), where N.sub.z is described above with respect to FIG. 3.
At time t.sub.0 guidance system output (215 and 300) commands a step increase in positive z-axis acceleration. Between times t.sub.0 and t.sub.1 the autopilot controller 225 sends a command signal 400 (corresponding to control signal 230) to actuate the missile's aft fins to deflect in a direction opposite the desired maneuver (sign convention denotes this as a positive deflection, refer to FIG. 1), producing an aerodynamic force on the fin surfaces in the negative z-direction. This force on the fins momentarily accelerates the missile's body in a direction opposite the commanded maneuver, thus introducing an inherent delay in the maneuver. Rotation of the missile's aft end causes the missile nose to pitch downward and the missile body to eventually accelerate in the positive z-direction. Between times t.sub.1 and t.sub.2, the autopilot controller 225 commands the aft fins to deflect in the same rotational direction as the maneuver, producing a fin force in the direction of the maneuver and causing the missile rotation to slow. After time t.sub.2, the autopilot controller commands a fin deflection in a direction opposite the maneuver, producing an opposing force and a rotation in the direction of the maneuver to maintain a steady rate and transverse acceleration in the positive z-direction. (Note, one skilled in the art will realize that this description also applies to accelerations in other directions.)
It is important to note that aft control mechanisms achieve missile accelerations by applying a force, initially, in the direction OPPOSITE the maneuver, see 410, which causes an inherent delay in the missile's response to the commanded maneuver, see 405.
2.3 Dual-Control Device Systems
The amount of control authority available to the missile is, in general, bounded by the length and diameter dimensions of the missile's airframe 100. Putting two control devices--dual-control strategy--on a single missile increases the amount of control moment which can be applied to the vehicle and, therefore, enlarges the missile's potential for increased maneuverability.
While some conventional missile control systems, such as the PAC-3 (Loral Vought Systems), employ both forward and aft control devices, they do not employ them in a cooperative manner for planar maneuvers. That is, the forward control device may be used to control the vehicle's pitch maneuver, while the aft control device may be used simultaneously to control the vehicle's roll motion.
A functional block diagram of an autopilot control system employing the dual-control concept is shown in FIG. 5. Block 200 represents the physical vehicle, including all vehicle subsystems such as control actuation, propulsion and inertial measurement systems. Output from the inertial measurement system (encoding, for instance, measured system roll, pitch and yaw rates and transverse accelerations) is shown as feedback signal 205. Measured vehicle response 205 is compared with a commanded dynamic response signal 215 from the guidance control system 210 to create an error signal 220 in a conventional feedback architecture. The dual-control autopilot controller 500 uses the error signal 220 to generate a forward control signal 505 and an aft control signal 510. It is the missile control signals 505 and 510 that control the missile's forward and aft control devices such as thrusters 110 fins 115.
2.4 An Intuitive Approach to Improving Missile Maneuverability
Dual-control of competitive devices has not heretofore been used in a cooperative manner because it is a challenging control problem. The difficulty in implementing a dual-control strategy lies in being able to allocate how much of the desired maneuver should be the responsibility of each control mechanism. That is, how much should the forward control device be actuated and how much should the aft control device be actuated to effect the commanded dynamic response.
Since the maneuverability of the missile is obtained via application of moments by the fore and aft control devices, intuition suggests that the fastest response using a dual-control strategy should be obtained by having the two control devices apply the largest controllable moment couple. In other words, an intuitive approach to improving a missile's dynamic capability is to simply use the individual forward and aft control strategies--the same command shapes as shown in FIGS. 3 and 4--but appropriately scaled. A control mechanism employing this approach is shown in FIG. 6. In response to a command signal 300 from the guidance control system 210 for a step increase in acceleration in the positive z-direction, the missile's autopilot controller 500 generates a thruster control signal (505 and 600) and a fin control signal (510 and 605) to effect the maneuver. Element 610 represents the missile's measured transverse acceleration response N.sub.z (corresponding to feedback signal 205) to the commanded maneuver, where N.sub.z is described above with respect to FIG. 3.
It is important to note that using this intuitive control strategy, the vehicle's acceleration in the commanded direction is delayed with respect to a isolated forward control strategy (compare 310 and 610). As previously noted, this delay is caused by the applied fin force being in a direction opposite that of the desired motion. It is recognized in the field that use of aft control devices introduce an inherent delay in missile response. Thus, the intuitive approach to improving a missile's dynamic capability using a dual-control strategy suggests that the command signals to fore and aft control devices be scaled in such a manner as to provide the desired acceleration.